1. Field of the Invention
This invention relates in general to antennas and more particularly, to portable line-of-sight array antennas.
2. Description of the Related Art
The ability to reliably communicate from one location to another is important in many situations, but few have the potential to be as important than those communications that occur between military personnel in the field of battle. Soldiers must be able to wirelessly communicate reliably and efficiently with others.
Wireless communication is accomplished through use of a radio connected to a radiating element, or antenna. An antenna is an impedance-matching device used to absorb or radiate electromagnetic waves into the environment. The function of the antenna is to “match” the impedance of the propagating medium, which is usually air or free space, to the source of the radio waves, i.e., output of the radio.
Antennas are available in many different shapes and sizes. The particular shape and size of an antenna designed for a particular application depends on many factors, such as the frequency or range of frequencies being received and transmitted, the expected environment the antenna will endure, size limitations, power efficiency limitations, application particulars, and many more.
Communication between two stations on the ground is most easily accomplished with radiating elements commonly called “monopoles” or “dipoles.” A dipole has two elements of equal size arranged in a shared axial alignment configuration with a small gap between the two elements. Each element of the dipole is fed with a charge 180 degrees out of phase from the other. In this manner, the elements will have opposite charges and common nulls, or points of no charge. A monopole, in contrast, has only one element, but operates in conjunction with a ground plane, which mimics the missing second element.
One of the characteristics of antenna transmission is “polarization,” which describes the physical plane the signal is transmitted. A dipole or monopole oriented in a vertical position, with reference to the horizontal orientation of the earth's surface, radiates signals in a straight line with a vertical polarization, know as line-of-sight (“LOS”). For a second antenna to receive maximum signal strength, it too must have a vertical orientation. As the receiving antenna is rotated away from vertical, its maximum receive power diminishes until the antenna reaches a horizontal orientation (perpendicular to the transmit antenna), at which time the maximum receive power reaches zero. It is therefore necessary to be able to orient a first antenna to match the orientation of a second antenna to maximize transmission and reception power, or “gain” between the antennas.
“Man-Pack” radios are mobile radios designed to be carried or worn on a person and are commonly used by Military or Paramilitary soldiers in the field on the move or at halt. A configuration of an antenna often found in Man Pack radios is a helical antenna, called a “helix”. In its simplest form, a helix is a conducting wire wound in the form of a screw thread and propagates radio frequency (RF) waves with a vertical polarization.
Helix antennas are attractive for Man Pack portable radio applications because of the antenna's relatively small size. The helix antenna axial length is shorter than the traditional resonant monopole, which is typically ¼ of a wavelength (λ) or a dipole antenna, which is typically ½λ. A normal-mode helix antenna length is very short (nL<<λ), typically only 0.1 λ.
The helix antenna, although useful, has several shortcomings. One shortcoming is a helix antenna is fixed in length and width dimensions and has an appreciable weight. To be portable, the helix antenna must be disassembled when carried and reassembled when deployed, which takes several minutes for each process. In combat situations, each minute is critical to avoid loss of lives. Another shortcoming is the helix antenna has a series loss resistance of the long spiral conductor is substantial, thereby consuming power. In portable mobile applications power is limited to what can be easily carried in the field.
Alternative to a helix antenna is a lightweight, compactable, telescoping antenna that is similar in construction to a telescoping antenna found on automobiles or cellular phones. Telescoping antenna systems, although useful, are not without their shortcomings. Once shortcoming is a telescoping antenna works efficiently only over a narrow band of frequencies. Effective communication in the field requires systems that function over a broad range of frequencies. The advent of multi-octave Man Pack broadband hand-held radios requires a broadband antenna. Therefore, the standard telescoping antenna is incompatible with the new generations of broadband radios.
Accordingly, a need exists to overcome the shortcomings with the prior art and to provide a portable, lightweight, efficient, high gain, broadband, line-of-sight antenna communication system that can easily be deployed in the field.